1. Field of the Invention
The present invention relates to a substrate polishing apparatus having a substrate measuring device utilizing light, and more particularly to a substrate polishing apparatus capable of reducing influence on a polishing process from substrate measurement and improving measurement accuracy of a substrate measuring device.
2. Description of the Related Art
In a semiconductor fabrication process, a substrate polishing apparatus is used to polish a surface of a substrate such as a semiconductor wafer to a flat mirror finish. The substrate polishing apparatus has a polishing table, and a substrate is pressed against a polishing surface on the polishing table. Then, while a polishing abrasive is supplied onto the polishing surface, the polishing table is rotated to polish the substrate. There has been proposed a substrate measuring device utilizing light as a device for measuring a film on a substrate during polishing the substrate. For example, a film thickness can be measured to determine an end point of polishing based on the measured film thickness.
There has been proposed a stream-type device as this type of substrate measuring device. For example, Japanese laid-open patent publication No. 2001-235311 (Patent Document No. 1) discloses a substrate measuring device having a water supply passage provided in a polishing table. An outlet of the water supply passage is provided in the polishing surface, and pure water is ejected through the water supply passage to the substrate. Two optical fibers are disposed in a stream. Measurement light is emitted through one of the optical fibers to the substrate, and reflected light is received through the other of the optical fibers from the substrate. Then, the film thickness is calculated based on the reflected light.
The stream-type substrate measuring device supplies water into a through-hole formed in a polishing pad, so that slurry flowing into the through-hole between the polishing table and the substrate can be diluted, and that slurry attached to the substrate can be cleaned. Thus, influence on measurement from the slurry is reduced so as to maintain a required capability of measurement.
However, a large amount of water should be supplied in order to maintain a required capability of measurement. If water for measurement flows from the through-hole to a surface (polishing surface) of the polishing pad, then slurry is diluted. Dilution of the slurry may have an influence on the polishing performance.
The outflow of water will be described in greater detail. When the water supply passage is covered with the substrate, the amount of outflow of water is relatively small. However, in most of conventionally proposed substrate polishing apparatuses, a location of a substrate is positioned away from a rotational center of the polishing table, and the water supply passage is not covered with the substrate continuously. Specifically, according to the rotation of the polishing table, there alternately appear periods of time during which the water supply passage is covered with the substrate and periods of time during which the water supply passage is not covered with the substrate. During a period of time during which the water supply passage is not covered with the substrate, the amount of outflow of water is increased, thereby diluting the slurry so as to influence the polishing performance.
In the aforementioned substrate polishing apparatus, an opening is formed in the polishing surface to apply the measurement light to the substrate, as described above. The opening formed in the polishing surface should preferably be as small as possible so as to reduce the influence on the polishing process. In order to reduce the size of the opening, it is required to reduce a space for receiving the optical fibers. Under these circumstances, optical fibers having a small diameter have been used for film thickness measurement in a substrate polishing apparatus.
However, optical fibers having a small diameter emit and receive a small amount of light. Therefore, there has been desired a substrate polishing apparatus having an improved ratio of the amount of measurement light emitted to the amount of received light (light-receiving efficiency).